Tubulin binding agents and corresponding prodrug constructs

A compound of the formula:

• wherein:
• Two of R1a-e are methoxy and the remaining R1a-e are H,
• One of R2 through R6 is a methoxy,
• and at least one of the remaining R2 through R6 is selected from the group consisting of OH, halogen, amine, lower alkoxy, phosphate, phosphoramidate, or amino acid acyl, and the remaining R2 through R6 are H;
• and X is a single covalent bond.

A compound of the formula Ia:

• wherein:
• Two of R1a-e are methoxy and the remaining R1a-c are H,
• One of R2 through R6 is a methoxy,
• and at least one of the remaining R2 through R6 is selected from the group consisting of OH, halogen, amine, lower alkoxy, phosphate, phosphoramidate, or amino acid acyl, and the remaining R2 through R6 are H;
• and X is a carbonyl.

A compound of the formula:

• wherein:
• R1a, R1b, R1c, R1d, and R1e are independently selected from the group consisting of H, halogen, lower alkyl, or lower alkoxy,
• R2 through R6 are independently selected from the group consisting of H, OH, halogen, amine, lower alkyl, lower alkoxy, phosphate, phosphoramidate, or amino acid acyl;
• and
• X is a carbonyl group.